1. Field of the Invention
The present invention relates generally to asymmetric bidentate silane reagents for modifying the surface of liquid chromatography supports, these surface modified supports and methods of making and using the same.
2. Discussion of Related Art
Liquid chromatography is an analytical method used to separate one or more species, such as ions or solutes, present in a carrier phase (i.e., solution) from the remaining species in the carrier phase. The carrier phase is typically passed through an open column packed with a granular material, commonly referred to as the support material. The most common support materials are silica-based.
The rate at which a particular solute passes through a chromatographic column depends upon the affinity of the solute for the support material, with solutes having a higher affinity for the support material passing through the column at slower rates. In certain cases, the affinity of a particular solute for the support material may be so high that essentially none of it passes through the column, resulting in substantially complete separation of that solute from the remaining species present within the carrier phase.
In large part, the affinity a solute has for a support material depends upon the functional groups present at the surface of the particles of the support material. Therefore, in many circumstances, it is desirable to bond modifying reagents containing certain functional groups to the surface of these particles to manipulate the rate at which different species pass through the column. For example, to separate an anionic species present in a carrier phase from a cationic species present in the carrier phase, a modifying reagent having cationic functional groups may be bonded to the surface of the particles of support material.
In addition to the functional groups contained within a surface modifying reagent, the strength of the bond between the reagent and the surface of a support material (i.e., stability of the modifying reagent) should be considered when choosing the reagent. If the bond between the modifying reagent and the support material surface is not strong enough, the modifying reagent can be released from the substrate during a liquid chromatography experiment. This can contaminate the separation product and/or reduce the efficiency of separation. Moreover, the ability of the modified support material to be re-used can be limited. Generally, a surface modifying reagent that bonds to the surface of a support material through two reagent atoms is more stable than a similar surface modifying reagent that only bonds to the surface through one reagent atom.
Still another factor to be considered when selecting a surface modifying reagent is the molecular structure that the reagent exhibits after it is bound to the surface of the support material. For example, the inherent variation of certain chemical and physical properties of polymers can result in problems with predicting the affinity of a given species for support materials having polymeric surface modifying reagents bonded thereto. Moreover, polymeric surface modifying reagents can cause problems with mass transport of a carrier phase through a liquid chromatography column, decreasing the efficiency of separation.
Silanes are the most commonly used surface modifying reagents in liquid chromatography. For example, "An Introduction to Modern Liquid Chromatography," Chapter 7, John Wiley & Sons, New York, New York 1979; J. Chromatogr. 352, 199 (1986); J. Chromatogr., 267, 39 (1983); and Advances in Colloid and Interface Science, 6, 95 (1976) each disclose various silicon-containing surface modifying reagents. However, these reagents bond to the surface of support materials through only one silicon atom, reducing the stability of these substrate-reagent bonds.
U.S. Pat. Nos. 3,722,181 and 3,795,313; EP Patent Application 129,074; J. Chromatogr. 298, 389 (1984); and Angew. Cheme. Int. Ed. Engl. 25, 236 (1986) each disclose polymeric silicon-containing surface modifying reagents, but, according to U.S. Pat. No. 4,746,572, discussed below, the use of these reagents has resulted in problems with mass transport of carrier phases through columns. In addition, these columns can demonstrate a lack of predictability with respect to the affinity of certain species for the support materials, according to U.S. Pat. No. 4,746,572.
U.S. Pat. No. 4,746,572 discloses bidentate silanes for use as surface modifying reagents in liquid chromatography. This reference emphasizes the use of symmetric bidentate silanes. However, as demonstrated in examples disclosed below, support materials having symmetric bidentate silanes bound thereto can demonstrate nonoptimal performance in some liquid chromatography separations.
From the foregoing discussion, it is clear that, despite the extensive efforts made in the art to provide improved surface modifying reagents for use in liquid chromatography, there remains a need to provide such reagents that exhibit improved stability under experimental conditions and predictability with respect to affinity to species in carrier phases. It would be advantageous for such reagents to be readily synthesized and easily bound to the surface of support materials.